The built in voltage gauges are only approximately accurate. If you want to know what is really going on, buy even a cheapo HF digital voltmeter (about $5) or splurge and pay $20 to $25 for a better one at Sears or Home Depot or Lowe's. Use it by connecting it directly to the battery, and see what it says then. Depending on temperature, and state of charge of the battery, you should see in the range of 13.6 to 14.4 volts when the engine is running.
For checking while driving, make up a cigarette lighter plug with wires for the meter, and monitor the voltage that way. Plug it into the dash lighter outlet. It will not be as stable, and the voltage may not be quite as high, due to the resistance in the wiring of the rig, but once you see what is "normal" under various conditions, you can get a better idea of what you can expect from your specific vehicle.